1. Field of the Invention
This invention relates in general to thermoplastic insulation for conduits and inparticular to a thermoplastic outer pipe with an annular layer of foamed insulating material.
2. Description of the Prior Art
Conduits for carrying chilled water, such as used in building complexes, should be thermally insulated. In addition, conduits exposed to cold atmospheric temperatures must be thermally insulated.
One known method of thermally insulating a section of conduit is to place a thermoplastic outer pipe over the section of conduit, centralize it with standoffs, then blow foamed thermoplastic material, normally liquid urethane, into the annular space. Heat is generated in the foaming process, and once cooled, the foamed urethane is semirigid. The urethane is chemically bonded to the conduit and outer pipe in the sense that it adheres to the walls other than by friction, although it is not fused with the walls.
It is difficult with this technique to achieve uniformity in thickness of the annular layer. The heat generated tends to distort the inner and outer pipes, standoffs are difficult to retain in place, and voids often result. The resulting product thus fails to insulate as well as desired. In addition bursting strength may be lowered if voids result, since the bursting strength depends partially on even and uniform contact with the outer pipe, annular layer, and conduit.
In addition joining double walled insulated pipe while installing or laying is difficult and time consuming. The inner pipe ends and outer pipe ends are not joined simultaneously since they normally are not truly concentric. They are joined in separate operations with insulating material spliced in.